The present invention relates to a system relying on logic means implementing phase-shift modulation techniques useful in radio frequency transmission, and, more specifically to an arrangement of comparators, timing and logic means defining in cooperative assocation each with another or others Boolean logic directed processing of a modulating information bearing signal to be impressed on a carrier or fundamental frequency wave.
Various circuits are known and suggested to be employed in the transmission of an information bearing signal input to a system at a given relatively high frequency to be impressed upon or combined in some manner with a second relatively low frequency signal defining in combination an output of the system suitable for transmission. Techniques of the sort characterized are known variously as pulse width, amplitude, or, as in this instance, phase-shaft modulation techniques. Exemplary is the production of a pulse width modulated signal by varying the duration of the output voltage pulses of the two signals that are applied to an inverter circuit. Silicon controlled rectifiers are ideally suited to this mode of transmission and are employed to facilitate triggering action defining an on condition during a time interval equal to a half cycle of the voltage wave. An exemplary amplitude modulation circuit is described in U.S. Pat. No. 4,319,204 assigned to the assignee hereof. Therein disclosed is an exciter used with a pair of amplifier circuits having a common output circuit producing the desired amplitude modulating output signal. The exciter is employed to control the relative phases of the control signals fed to a pair of amplifier circuits. Generally, the result of employing any such exciter system is production of two square-wave signals having a mutual phase relationship based upon a varying level input, such as an audio signal.
Disclosed in U.S. Pat. No. 4,346,354 likewise assigned to the assignee hereof, is a system for producing two rectangular wave signals whose pulse widths are based on an amplitude varying signal fed to the system wherein the two rectangular pulses can be combined and processed to reproduce the original amplitude varying signal. In accordance with an embodiment, an arrangement of switches are used to generate the pulses and are on only for the length of time necessary to produce the required pulse width and are off for the remainder of the time. Pulse width in radians of the rectangular pulses varies linearly so that the fundamental wave peaks vary as a sine function. The invention therein described shows that if pulses having a width of .eta. in radians are centered about 0 and .pi., the amplitude of the fundamental wave will be given by the curve generated by an acrsine function generator. The pulses are generated a great number of times for each small change in the curve causing the peaks of the fundamental wave to form an envelope which matches the amplitude of the normalized arcsine signal at all points between 0 and 1 (normalized).